Field of the Invention
The invention relates to a pregripper of a sheet-fed printing press having a drive unit for pivotally or swingably driving a pregripper bar secured to a pregripper support device, the drive unit having main and auxiliary cam elements disposed on a drive shaft so as to be fixed against rotation relative thereto, and cam follower members cooperatively engaging the cam elements so as to produce a movement of the pregripper bar.
Pregrippers of this general type have become known heretofore. They serve to grip a sheet of paper aligned by front and side lays and resting on a delivery table, and to accelerate the sheet to the press speed of the sheet-fed printing press. For this purpose, the pregripper has a pregripper bar which carries a number of mutually spaced-apart, row-wise arranged grippers. The grippers grip the sheet along the leading edge thereof and accelerate it as a result of the pivoting or swinging movement of the pregripper bar. The sheet is released approximately in the middle of the pivoting movement, which is at press speed. The pregripper bar swivels back into the starting position thereof so that it can grip the next sheet. To perform the pivoting or swiveling movement of the pregripper bar, a drive unit is assigned thereto which has main and auxiliary cam elements disposed thereon so as to be fixed against rotation relative thereto. These cam elements are set into rotary motion by the drive shaft and thereby deflect cam follower members moving along therewith. The cam follower members are formed as cam rollers, for example. These cam rollers are part of a transmission member, which is connected to a pregripper shaft so as to be fixed against rotation relative thereto, so that the cam rollers impart the deflecting movements thereof as a pivoting or swiveling movement to the transmission member, which causes corresponding rotary motion of the pregripper shaft. Due to the prescribed outer contour of the main cam element, the pregripper shaft is rotated a given angle in the sheet acceleration direction. The accessory or auxiliary cam element rests on the cam roller assigned thereto and controls the motion, so as to attain a maximum freedom from play. The outer contour of the auxiliary cam element is selected so that it effects a reverse or return motion of the transmission member, after the sheet acceleration movement, thereby causing a reverse rotation of the pregripper shaft. During this reverse rotary motion of the pregripper shaft, the main cam element rests on the cam roller associated therewith, in order to prevent excessive play from occurring there as well. The pregripper bar is disposed by means of a pregripper support assembly on the pregripper shaft so as to be fixed against rotation relative thereto, so that the pregripper bar executes a pivoting or swiveling movement corresponding to the rotary motion of the pregripper shaft. During operation, the sheet-fed pregripper bar is thus pivoted reciprocatively at a cadence or cycle dictated by the main and auxiliary cam elements. A transmission of force for performing this pivoting or swiveling motion is effected from the transmission member to the pregripper shaft, and from the latter, via the pregripper support assembly, to the pregripper bar. The pregripper support assembly has two pregripper supports, which extend radially and are disposed on both end regions of the pregripper bar. Particularly at high press speeds, for example when several thousand sheets are being transported per hour, torsional vibrations of the pregripper shaft can occur, which cause oscillations, particularly bending oscillations, of the pregripper bar. A result thereof can be uneven or nonuniform gripping by the grippers of the pregripper bar when the sheets to be accelerated are engaged, that is, the gripping being not always at the same place and possibly not at the same time. Because of this uneven gripping, the danger arises that the sheets, oriented or aligned beforehand by the side and front lays, will be delivered askew to the printing unit. To counteract these torsional and bending vibrations, an attempt has been made to damp them by constructing the machine or press elements very massively and rigidly, particularly by providing an oversized pregripper shaft. This requires increased space, and calls for additional material and considerably more weight.